It is known that failures, complicated wiring, and frequent errors are often found in conventional mechanical type forward/reverse current electromagnetic switches for forward/reverse rotation motors. Such failures result in embarrasing situations in which motor becomes inoperable in the field when it is urgently needed. Before technicians come to the site to repair the motor, users can do nothing but complain.
There is one type of forward/reverse current electromagnetic switch presently common in the market, the "Integrally constructed forward/reverse current electromagnetic contactor" described in file number 74205969, filed in the to Taiwan Patent Office on Jul. 20, 1985, and published in the Taiwan Patent Gazette dated Jan. 1, 1986. However, this type of forward/reverse current electromagnetic contactor has the following disadvantages:
(1) As shown in FIG. 9, an analytical drawing of the mechanism of the electromagnetic contactor, the entire mechanism is floatable within a fixed base-plate without fixed location; similarly, its movable contact point member a is also floatable within and above the base-plate and there is a considerable gap between the movable contact point member and the base-plate. When a fixed iron core is excited by a coil and becomes capable of attracting a movable iron core C, one end of the movable contact point member a is downwardly pulled by a link B and causes the movable connect point member a to swing with a spring D as its pivot so that the contact point on the movable connect point member a may contact a fixed contact point. In the event the attractive force F comes from the left-hand side as shown in FIG. 9, the movable connect point member a is turned with the left-hand instantaneous center 01 as its fulcrum. Conversely, when the attractive force F comes from the right-hand side, the movable contact point member a is turned with the left-hand instantaneous center 02 as its pivot. Because there is no fixed center for the entire mechanism, because the structure of the contactor itself can not provide fixed contact points located in the same plane, and because the movable contact point member a is moved by different attractive forces F which are generated whenever there are currents of different voltages and amperes passing the coil, swinging of the contact point member a with a different fulcrum causes the contact point to contact different places, resulting frequently in poor contact of the contact point and causing the motor to run with a single wire and finally be burnt out or become inoperable. PA0 (2) As the mentioned above since the entire mechanism of the electromagnetic contactor is floatable, when the contactor has been used for a period of time, or is used for a larger capacity motor to generate larger current to pass through the contact point, either contact point of the movable contact point member a is likely to become stuck to the fixed contact point and be retained at the position of 02 as shown in FIG. 9. If another reverse current push-button switch is pressed at this point, so that another coil excites the fixed iron core to cause the contact point of the movable contact point member a to contact the fixed contact point, then a short-circuit will occur and the electromagnetic contactor will burn out. In a less serious situation, in which the electromagnetic contactor for controlling a motor is installed in a control box, other electrically-controllled components may also be damaged by such a burn out, which not only stops operation of the machine itself but also could affect the running of a whole plant. In a much more serious situation, in which the electromagnetic contactor is used for a motor operator of a rolling shutter which is installed in a combustible environment, such as in and above an interior ceiling, a fire might occur and cause death and injury of persons and other serious physical damage and losses.
It therefore has been an objective of the inventor to provide a seesaw type mechanically interlocked electromagnetic switch for controlling forward/reverse current system which eliminates the disadvantages exhibited by in the prior art systems.